Numerous criteria must be met by an alloy to be used in the fabrication of prosthetic dental appliances such as procelain-veneered fixed bridgework and crowns. For example, the alloy must be tissue tolerant, tarnish resistant, corrosion resistant and non-toxic.
In addition, the alloy should form a protective and adherent oxide on its surface during torch melting and during the porcelain-firing cycle which does not grow dramatically in thickness. The oxides formed must be compatible with the porcelain; otherwise, they may affect the thermal expansion of the interfacial porcelain. Still further, the oxides should not discolor the porcelain. Most preferably, the oxide should be able to bond the porcelain to the alloy without the need for a separate bonding agent.
The alloy must also have a coefficient of thermal expansion slightly higher than that of the porcelains currently available on the market thereby placing the porcelain under compression and minimizing the stresses formed at the interface.
The alloy also should be shape-stable with porcelain application, possess adequate strength for function, produce an acceptable fit and be solderable. Finally, it should possess a high modulus of elasticity, high-yield strength and hardness and be easily cast, ground and polished using techniques conventionally employed in dental laboratories.
The criteria which govern the selection of a suitable alloy for use in the preparation of porcelain-veneered fixed bridgework and crowns are quite different from the criteria involved in selecting alloys for use in the fabrication of partial dentures which generally are not used in conjunction with porcelain. These criteria, to a large extent, have heretofore been met by alloys having a high precious metal content. Such alloys have contained gold, platinum, palladium, silver, indium, tin, gallium, zinc, and the like, and trace metals. Formulations of alloys of this type are set forth in U.S. Pat. Nos. 1,283,264, 3,413,723, 3,667,936, 3,767,391, 3,819,366, 3,981,723 and 4,007,040.
With the ever increasing and fluctuating cost of precious metals and the superior physical properties and technological advantages offered by nickel-chrome-base alloys, such alloys have become widely used as an alternative to precious alloys in dentistry. These alloys generally utilize tin, gallium and the like to impart specific physical characteristics. Typical of such alloys are those described in U.S. Pat. Nos. 2,089,587, 3,304,177, 3,464,817, 3,749,570 and 3,914,867.
Currently, there is growing concern about nickel being an allergen and beryllium being a toxic element. Although much data are still needed, there is an apparent need for an alloy which contains neither nickel nor beryllium, has a low precious metal content and yet meets the above criteria.
A number of cobalt-chromium base alloys with and without nickel and/or beryllium have heretofore been employed in dentistry for the fabrication of removable partials, crowns and bridgework. Typical of such alloys are those described in U.S. Pat. Nos. 3,756,809, 3,802,875, 3,802,934 and 3,837,838. However, their compositions and physical and thermal properties have limited their use for porcelain-veneered crown and bridgework.
Cobalt-chromium based alloys having a variety of compositions and said to be useful for porcelain-fused-to-metal restorations have been disclosed in U.S. Pat. Nos. 4,229,215, 4,253,869, 4,255,190 and 4,263,045. Of these, U.S. Pat. Nos. 4,253,869 and 4,255,190 describe alloys including cobalt, chromium and ruthenium. None of the alloys disclosed in these patents, however, includes aluminum or aluminum/yttrium, which, in accordance with the present invention, has been found to significantly improve the oxide formed on the alloy and the reactivity of the alloy with the melting crucible and the casting investment.